FIG. 1 shows an existing dial-type timer 20. Locking pawl 22 protrudes from an upper portion of the face 20a of the timer 20. Locking pawl 22 engages with a setting ring 23 (FIG. 2) mounted on dial 21 by engaging with a groove 23a on the outer periphery of the setting ring 23. In this way pawl 22 fixes the position of setting ring 23 and the dial 21. On the inner periphery of setting ring 23 are pawls 23b, which engage in slots 21a on the outer periphery of dial 21. Rotation of the setting ring 23 and the dial 21 sets the operating time.
Setting ring 23 is used to make it easier to set the timer dial 21 when the same operating time is set repeatedly. It also prevents accidental missettings resulting from different workers performing the same job. Dial 21 is set to the position which corresponds to the repeated operating time by the rotation of setting ring 23 to a position where the locking pawl 22 engages with groove 23a of the setting ring. The setting ring 23 engages with the sawteeth on the outer periphery of dial 21 in such a way that both the dial 21 and setting ring 23 rotate in unison. When the setting ring 23 and the dial are rotated so that slot 23a engages the pawl 22, the desired time is set.
With an existing dial-type timer of this type, locking pawl 22 on the upper portion of the face of the timer gets in the way when dial 21 is used without the setting ring 23, making the operation difficult.
Additionally, when dial-type timers are used in process control, they are not repeatedly set, but are set to values which should be changed only within a fixed range. However, existing dial-type timers are incapable of preventing values from being set beyond the upper and lower limits of the desired range. Thus, when such a dial-type timer is used for process control, the operator must pay careful attention whenever he changes a set value and note whether or not the new value falls within the fixed range of permitted values. He is, then, unable to relax. He may also mistakenly set values which exceed the fixed range.
The circuit design shown in FIG. 3 is well known as a timing mechanism of the type conventionally used. In FIG. 3, time-limit circuit 101, which comprises an integrated circuit (IC), and output relay 102 are connected in parallel to DC power supply 103 through switch 104. The base of an output switching element 105 is connected to the output terminal of the time-limit circuit 101. This element 105 is switched on and off by the output from time-limit circuit 101. The output relay 102 is connected in series with the collector-emitter pathway of output switching element 105. A diode 106 used to prevent reverse flow of current.
In an existing timing device constructed as described above, current I0 is supplied by DC power supply 103 when switch 104 is thrown. A part of this current, I1, is supplied to time-limit circuit 101, and circuit 101 operates and begins timing. After a specified time has elapsed, the aforesaid time-limit circuit 101 determines that the time is up. Its output causes switching element 105 to turn on, and output relay 102 is operated by current I2.
With the existing timing device described above, current I0, supplied by DC power supply 103, must be equal to I1+I2, that is, to the sum of the current supplied to the time-limit circuit 101 and that supplied to output relay 102. It is thus necessary to use a power supply with a current large enough to supply total current I0. Further, the current I0 supplied by DC power supply 103 is liable to fluctuate significantly when output relay 102 switches on or off. A power supply must be used which has sufficient capacity so that the power supply voltage V0 does not fluctuate when output relay 102 goes on or off. If the power supply does not have sufficient capacity, it will prove difficult to supply stable power to time-limit circuit 101. Without stable power, the circuit 101 will malfunction and the timing will be inaccurate.